What Is Embankment And Why It Is Constructed?

An embankment is a man-made hill of compacted earth, stone, and soil used to elevate a roadway or railway above the ground level.

In dams, an embankment is made up of layers of materials like soil, sand, clay, or rock, with the most impermeable materials forming the core and more permeable substances on the upstream and downstream sides.

When designing an embankment, factors such as the height of the fill, the material used, and the potential for settlement and stability need to be taken into account.

If the fill height is particularly high and the foundation soil is weak, it may be necessary to improve the ground in order to prevent settlement of the foundation soil.

Once the embankment has been constructed, further settlement may occur due to consolidation of the fill or settlement of the fill height, or both. In this article, we will cover the different characteristics, properties, types, and tests used to evaluate embankment materials.

What Is The Purpose Of Embankment?

An embankment is a structure built to hold back water, such as a river, and protect the land on either side from flooding.

Embankments have been used for thousands of years and continue to be important in modern times for protecting against flooding and for various other purposes.

Characteristics of Embankment Materials

In order to be used in the construction of embankments, the material must meet the following requirements as outlined by the U.S. Bureau of Reclamation’s Earth Manual:

Fine-Grained Soil

It must be a fine-grained soil that is homogenous and free from any potential pathways for water to pass through;
It must be impermeable to prevent excessive water loss;
It must be able to withstand the weight of additional layers of material without excessive consolidation; it must have a high shear strength.
It must not become excessively soft or consolidated when saturated with water.

Coarse-Grained Soils

Coarse-grained soils should be made into a smooth, uniform mixture without large empty spaces.
They should drain well and not compress too much under additional weight.
These soils should also have a strong resistance to shearing.

Material Properties of Embankment

Embankments should be made of the following properties of materials;

Gradation

Gradation refers to the range of particle sizes in a soil or fill material. A well-graded material typically contains a mixture of both granular and fine-grained soils, with particle sizes typically ranging up to 100mm.

Unit Weight and Specific Gravity

The unit weight and specific gravity of a material can impact the load that is transmitted to the underlying soil supporting an embankment. There are no specific requirements for a minimum or maximum unit weight before or after compaction.

Shear Strength

The shear strength of earthen fill material is important in determining the slope stability of an embankment and can be determined through triaxial compression or direct shear testing.

Compressibility

The compressibility of an embankment material is influenced by factors such as shear strength, compaction, void ratio, permeability, and saturation.

Permeability

Permeability refers to the ability of compacted fill material to allow for the drainage of excess moisture.

Types of Embankment Materials

There are three types of Embankment Materials, these are;

Fine-Grained Soil

Fine-grained soil that is used in embankments has low permeability, low shear strength, and high compressibility. This type of material has a high pore pressure due to rapid construction activities, which can lead to reduced shear strength and unstable conditions during construction.

However, fine-grained soil for embankments can be highly resistant to earthquake damage when used in the construction of dams and bunds, as long as the soil properties and placement conditions are appropriate.

Coarse-Grained Soil

Coarse-grained soils, typically made up of sand and gravel, are often used in structural fill zones and specialty filter and drain zones within embankments. They can also be used in core zones, particularly when they have more than 20% fines.

Coarse-grained soils that have less than 5% fines are permeable, easy to compact, and are not significantly affected by changes in moisture. However, they are prone to surface erosion due to wave action and surface runoff.

Broadly Graded Soils

Broadly graded soils have a diverse range of particle sizes and behave mechanically between fine-grained and coarse-grained soils. They generally have lower hydraulic conductivity, higher shear strength, and less compressibility compared to fine-grained soils.

Broadly graded soils that are found in colluvial and bouldery alluvial deposits are often used as material for embankments due to their high resistance to earthquake damage. These types of soils are significant sources of embankment material.

What Is Embankment And Why It Is Constructed?